1. Field of the Invention
The present invention relates to a thermal developing photosensitive member capable of forming an image by a dry process and an image forming method using the thermal developing photosensitive member.
2. Description of the Prior Art
A sliver-salt photographing method using a silver halide is a recording technology exhibiting excellent sensitivity and gradient expression characteristics and therefore used widely. However, the method involving to perform the process after exposing the image to light in a wet manner has suffered from unsatisfactory working property, convenience and safety.
On the other hand, a study of dry material, by which the wet process can be omitted, has been made and resulted in disclosures in Japanese Patent Publication No. 43-4921 and Japanese Patent Publication No. 43-4924. The foregoing disclosure each uses a photosensitive silver halide in a quantity as a catalyst and a non-photosensitive organic silver salt is used as an image forming material. The reason why the organic silver salt serves as the image forming material is considered to be due to the following mechanism.
(1) Exposure to an image causes a latent image to be formed on the photosensitive silver halide in a quantity which acts as a catalyst; and PA1 (2) The latent image serves as a catalyst and the photosensitive member is therefore heated, causing the organic silver salt and a reducing agent to take part in redox reactions to reduce the organic silver salt into silver which forms the image. PA1 Sensitizing Agent (a): merocyanine dye having a structure in which a thiazole nucleus or selenazole nucleus and hydantoin nucleus, thiohydantoin nucleus or selenohydantoin nucleus are combined with each other by a combining group having a methine group. PA1 Sensitizing Agent (b): merocyanine dye having a structure in which oxazole nucleus, thiazole nucleus or selenazole nucleus and hydantoinilidene-hydantoin nucleus having four substituents, hydantoinilidene-thiohydantoin nucleus having four substituents or hydantoinilidene-selinohydantoin nucleus having four substituents are combined with each other by a combining group having a methine group. PA1 Sensitizing Agent (c): merocyanine dye having a structure comprising two oxazole nuclei, two thiazole nuclei, or oxazole nucleus and thiazole nucleus at two ends thereof and further comprising, between the nuclei at the two ends, one nucleus selected from a group consisting of oxazolidone nucleus, thiazolidone nucleus and dihydroimidazolidone nucleus.
The heat developing photosensitive member has an advantage that it forms an image by a dry process in place of the wet process. Therefore, it has been used as an industrial photosensitive member in a variety of fields, for example, image communication, medical care and computer output fields. The heat developing photosensitive member exhibits excellent photosensitivity because the silver halide is contained as the photosensitive element. Furthermore, sensitization to a visible region can easily be performed.
Recently, a semiconductor laser has been developed and utilized which exhibits low cost, small size, light weight and excellent output efficiency as compared with a gas laser. Therefore, use of the low-cost, small and light laser beam source and the heat developing photosensitive member will enable a low-cost, compact and high performance dry image recording system to be realized.
A conventional gelatin-type silver halide photosensitive member for a wet process has employed a cyanine dye for the purpose of improving the photosensitivity with respect of long wave light, particularly, red light. However, the cyanine dye suffers excessively unsatisfactory sensitizing efficiency with respect to the heat developing photosensitive member for the dry process and therefore it has been considered to be inadequate for the foregoing use.
Although the silver halide photosensitive member for the wet process is able to decolor a sensitizing dye by the wet process, the conventional heat developing photosensitive member cannot decolor it sufficiently in a heating process and therefore raises a problem in that an image having a lower optical density cannot easily be obtained. If the sensitizing dye is decreased in quantity in order to lower optical density, problems of unsatisfactory photosensitivity and defective resolution arise.
The conventional heat developing photosensitive member has another problem in that the photosensitivity deteriorates excessively if it is stored in an unused manner for a long time.